The present invention related generally to measurement probes and more particularly to a high frequency measurement probe providing signal control for an electrical over stress (EOS) and electrostatic discharge (ESD) protection module.
Ultra high speed sampling heads used in time domain reflectometry typically dictate extremely low parasitic capacitances. This introduces unique problems. Sampling devices are much more sensitive to static charges residing on a device under test before a test probe touches it. The small geometry of the sampling diodes in the sampling heads often dictate low breakdown voltages. The low parasitic capacitance at the sampling head input means that for a given device under test (DUT) static charge, there will be a higher transient voltage at the sampler input because of the reduced charge sharing effect. It is therefore important to neutralize any static charge on the device under test before the sampling head input is coupled to the device under test.
Conventionally, users are advised to take all anti-static precautions including purchasing and installing antistatic equipment and employing anti-static procedures. Such equipment and procedures include using ionized airflow devices to reduce the DUT static charge on isolated conductors, ground straps on the test bench and the operator, and an anti-static mat around the test bench. Another piece of anti-static equipment that may be used with sampling heads is the SIU600 Static Isolation Unit, manufactured and sold by Tektronix, Inc., Beaverton, Oregon or the Model 1201 Static Isolation Unit, manufactured and sold by Picosecond ATE, Inc. Beaverton, Oreg. Referring to FIG. 1, there is representatively shown the static isolation unit 10 that includes an interface box 12, foot pedal 14, and a power adapter 16. The power adapter 16 is connected to a standard electrical outlet to provide DC power to circuitry within the interface box 12. An RF probe 18 for probing a device under test 20, such as circuit runs 22 on a circuit board 24, is connected to the interface box 12. A coaxial cable 26 couples the interface box 12 to a TDR sampling head 28 mounted in a sampling oscilloscope 30. The foot pedal 14 is connected to the interface box 12 for coupling the output of the device under test 20 through the interface box 12 to the sampling head 28. When the foot pedal 14 is in the normal position (not pressed), the input of a buffer circuit is coupled to a TTL logic high that cuts off current flow in a drive circuit to an RF relay in the interface box 12. The normally open RF relay coupled the probing tip of the RF probe 18 to electrical ground through a 50xcexa9 termination resistor 32. Positioning the probing tip of the RF probe 18 on the DUT 20 discharges any static charge stored in the DUT 20. Pressing the foot pedal 14 closes a low resistance switch and allies a TTL active low signal to the buffer circuit that activates drive circuitry in the interface box 12 that energizes the relay and connects the probing tip of the RF probe 18 to the sampling head input, allowing a measurement to be made. The circuitry in the interface box 12 operates under TTL active low logic allowing the foot pedal 14 to be replaced with a TTL external source. The use of TTL active low logic requiring the use of a low resistance switch in the foot pedal 14 for proper operation of the interface box 12 circuitry.
Proper use of the static isolation unit 10 prevents ESD and electrical over stress (EOS) static charge from damaging or destroying the sampling head. The main difference between ESD and EOS is that EOS can occur at a much lower voltage level that ESD. ESD static voltages are typically several hundred to several thousand volts, whereas EOS static voltages may be as low as 15 to 30 volts. The sampling diodes in the sampling head has a breakdown voltage of approximately 9 volts. EOS static discharge causes microscopic damage to the semiconductor layer of the sampling diodes in the sampling head providing a leakage current path around the semiconductor Schottky junction. Over time, the incremental damage of each occurrence of the EOS static discharge continues to degrade the performance of the semiconductor device until the leakage current causes excessive measurement error.
In TDR measurements of a device under test, an operator places the RF probe on the test point with operator""s foot off of the foot pedal 14. The probing tip of the RF probe is coupled to electrical ground through the interface box. Once the operator has properly placed the probe on the test point, the operator depresses the foot pedal with his or her foot and circuitry in the interface box couples the probing tip of the RF probe to the sampling head circuitry. After the measurement is made, the operator removes his or her foot from the foot pedal before removing the RF probe from the test point to disconnect the probe from the sampling head and reconnect the probe to electrical ground. However, in a production environment where repetitive probing is done by the operator, an operator may accidentally keep the foot pedal 14 depressed while repositioning the probe or moving the probe from one test point to another. This allows ESD and EOS voltages on the device under test to be coupled to the sampling head causing damage to the sampling diodes.
One solution is to move the low resistance switch in the foot pedal into the RF probe 18. This would result in a bulkier probe design requiring the placement low resistance switch in the probe along with a hand operated mechanical actuator to allow an operator to activate the switch for measurements. Such a design does not eliminate the possibility of an operator inadvertently keeping the low resistance switch closed while moving from one test point to another.
What is needed is a fail-safe electrostatic discharge and electrical over stress static discharge solution that prevents electrostatic discharges and electrical over stress static discharges from a sampling head input. Activation of the EOS and ESD protection should be incorporated into the measurement probe thus eliminating the need for a foot pedal. The measurement probe should automatically provide EOS and ESD protection for and signal connectivity to sampling head through the ordinary use of the probe for making measurement.
Accordingly, the present invention is a measurement probe providing signal control to an electrical over stress and electrostatic discharge protection module for coupling the measurement probe to input circuitry of the measurement test instrument. The measurement probe has a spring loaded coaxial probe assembly formed from a semi-rigid coaxial cable. The semi-rigid coaxial cable has a probing tip at one end and a threaded connector at the-other end for receiving a coaxial cable coupled to the electrical over stress and electrostatic discharge protection module. A compression spring is positioned on the semi-rigid coaxial cable for biasing the spring loaded coaxial probe assembly. A housing having an internal cavity extends the length of the housing and is exposed at opposing ends of the housing. The spring loaded coaxial probe assembly is disposed within the internal cavity with the probing tip extending from one end of the housing and the threaded connector extending from the other end of the housing. The housing is movable from a first position to a second position relative to the spring loaded coaxial probe assembly from pressure applied to the probing tip of the measurement probe in contact with a device under test. A pressure sensor has a first electrically conductive contact secured and electrically coupled to an outer shielding conductor of the semi-rigid coaxial cable and a second electrically conductive contact positioned and secured within the internal cavity of the housing The second electrically conductive contact is coupled to the control module via an electrical conductor. The probing tip is coupled to electrical ground via the control module when the housing is in the first position. The probing tip is coupled via the control module to the input circuitry of the measurement test instrument when the housing is in the second position. The control module is responsive to an activation signal generated by the first and second electrically conductive contacts of the pressure sensor contacting each other by movement of the housing to the second position.
The housing has first and second members with each member having a channel formed therein for receiving the spring loaded coaxial probe assembly and the pressure sensor. The channels form the internal cavity when the first and second members are joined together. The first and second electrically conductive contacts of the pressure sensor are disposed in a pressure sensor chamber that is a part of the internal cavity. The housing is preferably formed with an electrical conductor channel with one end of the electrical conductor channel extending to a pressure sensor and the other end extending to the threaded connector end of the housing for receiving the electrical conductor.
The first electrically conductive contact of the pressure sensor may be formed of a rectangular block of electrically conductive material having a bore formed therethrough for receiving the semi-rigid coaxial cable. The first electrically conductive contact may also be formed as a disk of electrically conductive material having a bore formed therethrough for receiving the semi-rigid coaxial cable. The second electrically conductive contact of the pressure sensor may be formed of a block of electrically conductive material having the electrical conductor coupled thereto. The second electrically conductive contact may also be formed of a flat conductor having a first portion exposed in the pressure sensor cavity and a second portion disposed within the electrical conductor channel. The second electrically conductive contact may further be formed as a fiat electrically conductive disk positioned in notches formed in the pressure sensor chamber. The flat electrically conductive disk as an oversized bore therethrough that allows for the free movement of the semi-rigid coaxial cable through flat electrically conductive disk. The second electrically conductive contact may further have a bore therethrough for closely receiving an insulating bushing. One end of the second electrically conductive contact extends past one end of the insulating bushing and the other end abuts an outwardly extending flange formed at the other end of the insulating bushing. The insulating bushing has an oversized bore formed therethrough that allows for the free movement of the semi-rigid coaxial cable through the bushing. The flange of the insulating bushing is secured to the housing.
The electrical conductor is preferably an insulated electrical wire. The insulation on the end portion of the insulated electrical wire may be removed to expose the electrical wire with the end of the exposed wire extending into the pressure sensor chamber to form the second electrically conductive contact of the pressure sensor. The electrical conductor may consist of first and second insulated wire segments. The first wire segment electrically couples the second electrically conductive contact of the pressure sensor to an electrical contact of an electrical connector receptacle mounted on the measurement probe. The second insulated wire segment electrically couples an electrical contact of a first electrical plug to an electrical contact of a second electrical plug. The first electrical plug mates with the electrical connector receptacle mounted on the measurement probe and the second electrical plug mates with an electrical connector receptacle having at least a first electrical contact mounted in the control module.
One end of the compression spring may abut the first electrically conductive contact of the pressure sensor with and the other end of the pressure spring abutting a shoulder extending into the interior cavity. The compression spring may also abut a spring retention member secured to the semi-rigid coaxial cable with the other end abutting the shoulder extending into the interior cavity. The spring retention member has transverse notch formed in the outer shielding conductor of the semi-rigid coaxial cable with a wire wrapped around the semi-rigid coaxial cable in an overlapping manner and engaging the transverse notch. The overlapping portion of the wire is soldered together to secure the compression spring retention member to the semi-rigid coaxial cable.